FIG. 1 schematically illustrates an optical transmitter 2 in which a Dual Parallel Mach-Zehnder (DPMZ) modulator 4 is used to generate a modulated optical signal for transmission through an optical communications system. In the transmitter of FIG. 1, a narrow band light source 6 (such as a laser diode, for example) generates a narrow band continuous wave (CW) light which is supplied to an input of the DPMZ modulator 4. Within the modulator 4, the CW light is divided into two light paths, which are identified as the In-phase (I) and Quadrature (Q) branches. Each branch includes a respective internal Mach-Zehnder modulator 8 designed to modulate the light using a respective drive signal to yield a corresponding modulated branch signal. At the output 10 of the dual branch Mach-Zehnder modulator 8, the two modulated branch signals are combined to generate the modulated optical signal for transmission through the optical communications system.
In the transmitter of FIG. 1, the internal MZ modulators 8 are driven by respective In-phase and quadrature drive signals VI and VQ, which are generated by a digital synthesizer 12 in a manner known in the art. For example, the digital synthesizer 12 (which may be configured as an Application Specific Integrated Circuit) may operate to generate a set of digital in-phase and quadrature signals. In FIG. 1, these digital signals are indicated as SI and SQ. A respective Digital to Analog Converter (DAC) 14 may be used to convert the digital signals to analogue, which may be conditioned using analog variable gain amplifiers (VGAs) 16 and filters (not shown) in a conventional manner to produce the analogue drive signals VI and VQ needed to drive the DPMZ modulator 4.
In order to optimize performance of the DPMZ modulator 4, a controller 18 is typically used to generate a set of bias signals for controlling a bias point of each internal MZ modulator 8 and a phase relationship between the two branches. An optical tap 20 may supply a portion of the output light to a photodetector 22 which outputs a tap signal I that is proportional to the power level P of the modulated optical signal appearing at the output 10 of the DPMZ modulator 4. Typically, the controller 18 implements a set of feed-back control loops based on a tap signal I for calculating a respective bias signal VbI and VbQ for each internal modulator 8, and a phase bias signal VbP is used to control the phase relationship between the two modulated branch signals. The bias signals VbI, VbQ and VbP are normally calculated based on the response characteristics of the DPMZ modulator 4 and the desired modulation format of the output optical signal. For example, in order to generate Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM) symbol constellations, the bias signals VbI, VbQ and VbP are calculated to obtain a “Min/Min/Quad” bias condition of the DPMZ modulator 4, in which each internal MZ modulator 8 is biased at a minimum of its respective response function, and the phase bias signal VbP is set to maintain quadrature (i.e. a 90° phase difference) between the two modulated branch signals.
In addition to controlling the bias point of each internal MZ modulator 8 and the phase relationship between the two branches, it would be desirable to also determine and control the power balance between the two branches. This may be referred to as the I/Q power balance of the DPMZ modulator 4, and may be denoted as PI/PQ, where PI is the optical power level of the modulated In-phase optical signal, and PQ is the optical power level of the modulated quadrature optical signal. Controlling the I/Q power balance is important for maximizing the signal-to-noise ratio (SNR) since with an ideal transmitter and channel, for circular-Gaussian noise the SNR is maximized when PI=PQ. It is not possible to control the I/Q balance using the configuration in FIG. 1 and by just measuring average optical power as there is only one optical tap, so other techniques are required.
FIG. 2 schematically illustrates an optical transmitter 2 in which a pair of DPMZ modulators 4 are connected in parallel to produce a Quad Parallel Mach-Zehnder (QPMZ) modulator 24 which generates a polarization multiplexed optical signal for transmission through an optical communications system. In the transmitter of FIG. 2, a narrow band light source 6 (such as a laser diode, for example) generates a narrow band continuous wave (CW) light which is supplied to an input of the QPMZ modulator 24. Within the modulator, the CW light is divided into two light paths, which are arbitrarily designated as X-Pol. and Y-Pol. Each path includes a respective DPMZ modulator 4 which operates substantially as described above with reference to FIG. 1. Thus, in the example of FIG. 2, the light in the X-Pol path is divided into I and Q branches, and modulated using respective drive signals VXI and VXQ. The two modulated branch lights (having average power levels of PXI and PXQ, respectively) are then combined to yield a corresponding X-Pol output light having a power level of PX. Similarly, the light in the Y-Pol path is divided into YI and YQ branches, and modulated using respective drive signals VYI and VYQ, The two modulated branch lights (having average power levels of PYI and PYQ, respectively) are then combined to yield a corresponding Y-Pol output light having a power level of PY. The X- and Y-polarization lights output from each DPMZ modulator 4 are then combined using a polarization beam combiner 26 to yield a polarization multiplexed optical signal at the output of the QPMZ modulator 24. A polarization rotator 28 may be used to ensure that the X- and Y-polarization lights are linearly polarized and are orthogonal to each other at the respective inputs of the polarization beam combiner 26. As in the example of FIG. 1, a controller 18 may use tap signals IX and IY from each polarization in order to optimize performance of the respective DPMZ modulator 4. In addition, an external tap 30 and photodetector 32 is used to obtain a corresponding external tap signal Iz that is proportional to the power level Pz of the polarization multiplexed optical signal output from the QPMZ modulator 24. FIGS. 3A and 3B illustrate an X-polarization response of the QPMZ modulator 24, in which X-polarization tap signal IX is shown on the vertical axis and the external tap signal IZ is shown on the horizontal axis. FIG. 3A shows an idealized response for non-inverting taps 20, infinite extinction ratio and zero leakage. FIG. 3B shows a response of a real transmitter (ie. non-zero leakage, finite extinction ratio etc.) with non-ideal taps, such as inverting taps.
In general, it would be desirable to be able to determine and control the power balance between each of the internal modulators 8 of the QPMZ modulator 24. Using the notation of FIGS. 2 and 3, it would be desirable to determine and control the relationship between each of the branch optical power levels PXI, PXQ, PYI and PYI. However, the same difficulties discussed above with reference to FIG. 1 also arises in the QPMZ modulator 24. In the presence of non-ideal performance, the power balance cannot be accurately determined by applying a dither to a drive signal and then measuring the corresponding response in one of the tap signals IX, IY or IZ. This problem is compounded in cases in which the taps 20 are inverting.
Techniques that overcome at least some of the aforementioned limitations of the prior art remain highly desirable.